Electrical machines have widespread use throughout industry. Most applications energize an electrical machine from fixed frequency voltages so that the machine is provided with a sine wave power while operating at a relatively fixed speed.
In many applications, it is desirable to vary the operating characteristics of the electrical motor such as its speed. Generally, variable speed operation in induction machines requires simultaneous variation of both the applied voltage and the applied frequency. Historically, this has been accomplished with special rotating machines between the induction motor and the power supply.
In recent years, high power semi-conductors have been utilized to provide the necessary voltage and frequency variations. Two solid state power conversion techniques generally available for the induction machine drive system are (1) dc voltage-fed forced-commutated inverter and (2) dc current-fed forced-commutated inverter. Both of these techniques utilize a two-step power conversion process by conversion of the fixed frequency and fixed voltage to a dc power link and conversion of the dc link power to variable frequency power for application to the induction machine.
Three primary disadvantages associated with either of these two conventional techniques are: (1) a relatively complex and costly inverter configuration using, in addition to the main switching devices, a multiplicity of auxiliary devices for main device turnoff and reactive current flow, (2) relatively high values of harmonic current flow in the induction machine unless even more complex switching patterns are used by the inverter. The harmonic currents create undesirable losses in the induction machine without creating useful output power, and (3) steep voltage wavefronts are applied to the induction machine which requires that special techniques be used for insulating the machine windings.
Similarly, the use of synchronous machines as variable speed drive systems are common. The synchronous machine field can be controlled by an auxiliary power supply. The technique of utilizing synchronous machine winding potentials to perform turn-off of power supplies using SCR's is well known. However, such systems have disadvantages of low power factor, creation of pulsating torques, require extra insulation, and has undesirable harmonics.
The present invention is directed to various improvements in a semi-conductor control system for controlling three phase motors by varying the voltage and/or frequency without the disadvantages of the prior art systems.